1. Field of the Invention
The present invention relates to an electrophotographic image generating apparatus which generates an image by scanning a photosensitive member with an optical beam and to a calibration method therefor.
2. Description of the Related Art
An optical scanning, electrophotographic image generating apparatus forms a latent image on a photosensitive member by optical drawing, develops the latent image with toner, transfers a resulting toner image to paper, fixes the toner image, and thereby produces a printout.
Basically, the optical scanning image generating apparatus has a single light source. However, to produce a printout at high speed using a low-speed device, a number of light sources are used in parallel in a single optical system. This reduces operating frequencies of an electronic circuit's drive unit and a processing system. In the case of color image generation, the image generating apparatus has as many light sources as there are color components if it is configured to have separate optical systems for different color components.
The electrophotographic image generating apparatus performs two-dimensional image generation using two-dimensional scanning resulting from a combination of main scanning, orthogonal to a sub-scanning direction of the photosensitive member, and sub-scanning carried out by rotation and the like of the photosensitive member. Consequently, drawing accuracy is greatly affected by optical system's accuracy, which depends on mounting of an optical mechanism as well as by travel accuracy of a scan driver. To maintain the drawing accuracy at low cost, a method has been proposed which permits, to some extent, unevenness (so-called misregistration) such as a skewed and/or inclined track of main scanning resulting from low mechanical accuracy and corrects the unevenness on images. For example, Japanese Patent Laid-Open No. 2004-170755 discloses a method which measures magnitudes of inclination and skew of scanning lines by means of an optical sensor, corrects bitmap image data so as to cancel the inclination and skew, and generates a corrected image. The method disclosed in Japanese Patent Laid-Open No. 2004-170755 corrects misregistration using a scan line changing process and tone correction process. The scan line changing process is the process of canceling deviation of an actual scan line from an ideal scan line by shifting image data by the same amount in the opposite direction, where the ideal scan line is assumed to be a straight line on a surface of a photosensitive drum parallel to a rotation axis of the photosensitive drum. The tone correction process is the process of apparently correcting a deviation smaller than one line by providing a density gradient in a certain range in front of and behind a point at which the line deviates (scan line changing point). Since the method corrects image data, it eliminates the need for a mechanical adjustment member or adjustment process during assembly. Thus, the method makes it possible to downsize color image generating apparatus and deal with misregistration at low cost.
To convert image information by image processing using an electronic circuit or software instead of adjusting the optical system, amounts of correction for skew and distortion of main scanning are measured and digitalized at the factory. The digitalized amounts of correction are stored in a storage of each image generating apparatus and then referred to during image generation to correct the image for skew and distortion of main scanning.
However, the amounts of correction measured at the factory for the image generating apparatus can change if optical parts are worn or torn over time or replaced due to a failure. In that case, amounts of correction have to be measured anew. This is done in an end-use environment or by returning the image generating apparatus to the factory, resulting in high costs and thus rendering cost reduction of the image generating apparatus meaningless.